The instant invention relates to an industrial instrument and method for monitoring particulate matter which is borne in the gaseous medium of a stack, duct or the like with improved sensitivity and selectivity of detection.
Presently stacks, ducts and the like for industry are frequently monitored by optical transmissometers and nephalometers which measure the concentration of particles in a gaseous fluid by the analysis of a light beam transmitted through or reflected by the fluid. However, there are certain problems which arise wherein optical methods are not sufficiently accurate such as in a low-opacity stack which even during a crisis situation may not contain sufficient material to produce a reliably measurable opacity; in a process which produces particulates too small for optical scattering and absorption, or where the particles have unfavorable indices of refraction for optical effects; or in a stack containing both harmful and harmless emissions, both of which actuate optical devices.
The inventors in U.S. Pat. No. 3,808,344 disclosed methods and types of apparatus for the detection of airborne particulate matter wherein a particle strikes a hot surface and emits a burst of many ions which are detected electrically by means of their charges. In such invention the air was induced to move in a path containing the hot surface with the particles thus transported to the surface by means of a pump or fan. In certain industrial applications, particularly those involving movement of gas or air in a stack or duct, the hot surface may be placed directly into the flow--whereby the gas movement transports particles to the hot surface. The hot surface is normally a heated wire or ribbon which is operated at a temperature whereby infrared and visible radiation is emitted. The passage of gaseous fluid in which particulate matter is suspended over the hot surface tends to cool same. The power requirement for keeping the hot surface at a proper operating temperature is dependent on the rate of cooling by the passing gaseous fluid. Still further, variations in flow produce variations in the rate of cooling of the filament and, with a given or steady power input which heats the surface, the temperature is subject to variations with airflow variations. Yet further, with long periods of use, the hot surface is slowly oxidized and, if in the form of a wire filament which is heated by ohmic heating, as is often the case, the wire suffers a reduction in diameter. With such reduction, the resistance of the filament is increased which requires a change in the power level if the radiation and hence the temperature, from the filament is to remain constant.
It will thus be understood that a need exists for an instrument which can be utilized in an industrial environment which has the sensitivity and selectivity necessary for the measurement of gaseous borne particulates found in industrial applications, which is not to be adversely affected by variable gaseous flow rates and temperatures, and which maintains the ability to provide a constant measurement irrespective of operative influences which tend to cause changes in the accuracy of the apparatus with age.